Active materials for a positive electrode of an electric cell of high performance are required to have a high electromotive force, a high open circuit voltage and, in addition, a small overvoltage on discharge, a good flat characteristic of discharge curve and a large discharge capacity per unit weight when they are used in combination with an appropriate negative electrode. Furthermore, the active materials are required to be neither decomposed nor dissolved in the electrolyte of an electric cell and have to be stable for a long period of time.
It is said that a positive electrode containing, as the active material, fluorine, which is an element having the highest positive standard potential among all the elements can provide a small-sized, light-weight electric cell having a high voltage and a high energy density, when used in combination with an appropriate negative electrode. The active materials for a positive electrode of such an electric cell which have been widely studied include fluorides of copper, nickel, silver, etc., (CF).sub.n and (C.sub.2 F).sub.n. The copper fluoride and the nickel fluoride have a high theoretical energy density and exhibit an open circuit voltage as high as at least 3 V when used together with lithium metal as a negative electrode, but disadvantageously they are unstable in the electrolyte and the self-discharge is so remarkable that the discharge capacity is greatly decreased in a few days. Further, the overvoltage on discharge is large and the uniformity of discharge voltage is poor. Thus, the cells using the copper fluoride and the nickel fluoride as the active material for a positive electrode have not yet been of practical use. On the other hand, (CF).sub.n and (C.sub.2 F).sub.n are excellent active materials for a positive-electrode which have an open circuit voltage as high as 3.2 V and an excellent uniformity of discharge voltage and are stable in an electrolyte for a long period of time in combination with lithium metal as a negative electrode. However, with (CF).sub.n the overvoltage on discharge is large and a large potential drop at a high load discharge can be observed. With (C.sub.2 F).sub.n , the overvoltage on discharge is smaller than that of (CF).sub.n, and (C.sub.2 F).sub.n is accordingly more useful, but the time for recovering the open circuit voltage after a high load discharge is long. Thus, until now, optimum active materials for a positive electrode were still sought.
The present inventors have therefore made extensive studies to develop active materials of a fluorine type for the positive electrode of an electric cell having high performance and, as a result, have found active materials for the positive electrode which not only almost perfectly satisfy the above described requisites for the positive-electrode active materials of an electric cell of high performance but also exceed the conventional active materials for the positive-electrode in various performances for the electric cell.